air filter vs fuel economy (tricky question?) 11

[ivymike]

Perhaps this is a tricky question...

Does having a dirty air filter hurt the fuel economy of a modern gasoline-fueled car? (Obviously the parts stores would have you believe that it does)

If so, how exactly?

If it's inlet restriction, then would you say the effect is the same as modifying the throttle actuator such that the throttle opens slightly less for any given pedal input than it would otherwise have opened? If this is equivalent, would you expect to get better or worse fuel economy as a result of such a modification?

 

[Lou Scannon]

Oops! Forgot which forum I was in!
There's a real star for you cardoc.

 

[patprimmer]

Here here

Regards

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[franzh]

Rats, Cardoc! Just as I was composing my comments about mass air flow systems, my computer re-boots (Windows security installation) and when I check back in, your excellent response is there.

I agree totally as well. Good job.

Franz

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[drwebb]

. . . assuming the frictional heat doesn't melt the paint off the air filter.

 

[drwebb]

;-)

 

[Rockwood]

Now that this conversation has finished, time to add my 2.3 cents...

One thing I haven't seen (maybe I missed it?) is the effect the increased right-foot pressure has on the vehicle's fuel maps. In addition, that increased demand from the driver, combined with an automatic transmission, may result in later shifts.

I know that when tuning a racecar, the higher the voltage read by the TPS (throttle position sensor), the "safer" you go with on your fuel tables. In addition, the closer you get to torque peak, the safer you go on your tables. From what I have seen of OEM fuel maps, the manufacturers are doing the same.

Of course, the question is whether or not the increased right foot pressure is enough to jump it into the next step on the map, or even if the target A:F ratio changes based on O2 sensor readings.

But, I'm just an amatuer weekend warrior when it comes to this kind of thing. Great read either way.

 

[dalcazar]

Ok now I'm going to re-ignite the flames a bit for the sake of clarity...

I agree that a modern car will not lose fuel economy due to a dirty filter since the engine always uses the correct amount of fuel for the air coming in to keep the mixture where it needs to be.

In the cited example with the blocked off air filter, the sensors still work just fine so economy doesn't suffer, but as you restrict more and more of the filter area, just how much manifold pressure do you lose and at what point do you start to notice serious power loss? Was the part about someone not noticing a difference even with only 25% or less of the air flow directed only at fuel economy? it seems counterintuitive that the same person wouldn't notice a power loss from that restriction (then again if you lend your car to someone, they wouldn't know what the actual base power feels like in the first place)

 

[patprimmer]

Rockwood

See my post 09/04/08 about 4 or 5 down from top.

I may be out of date, but as far as I know, TPS on OEM systems normally only have 3 readings, closed for idle/overun, cruise and WOT.

Idle/overun position or closed throttle adjusts to cut fuel on overun, but cuts fuel in at certain rpm to maintain idle.

Part throttle or cruise gives normal economical drivable mixture.

WOT gives extra fuel for performance and/or durability.

You point re A/T change point is valid and was missed.

Regards

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[Rockwood]

"See my post 09/04/08 about 4 or 5 down from top."

Figures I should miss one of the key posts that started a lot of these discussion points.

"I may be out of date, but as far as I know, TPS on OEM systems normally only have 3 readings, closed for idle/overun, cruise and WOT."

Yeah, this is something outside my area of experience as I have never been in the unfortunate circumstance to tune an OEM system - I don't envy those who need to iron out every single part-throttle-hiccup while maintaining proper emissions/economy. When we tune, we just look for a target mixture that is safe and creates the most power. If there are part throttle hiccups, we address them as they come up, but these are relatively unimportant. In addition, if the car is turbocharged, you override the TPS reading at certain MAF sensor voltage readings because a turbo car can create large amounts of positive manifold pressure in the "cruise" range since it's just jamming air past the throttle body. In those situations, the cylinder pressure is much too high to safely attempt a stoich target mixture.

"Idle/overun position or closed throttle adjusts to cut fuel on overun, but cuts fuel in at certain rpm to maintain idle."

Given.

"Part throttle or cruise gives normal economical drivable mixture."

That's what I figured, but with the complexity of some of the newer systems, they may have specific targets based on TPS voltage. Even with the older systems, I wonder that if you have a TPS that outputs 0-4v, if 3.5+v is considered WOT, or just 4v, and is a dirty filter enough to get into that range (doubtful, the more I think of it).

Another wrench in the works would be electronic throttle bodies where the gas pedal is simply a torque demand pedal and has no direct affect on the throttle body.

 

[JSteve2]

dalcazar,
I don't believe power loss to be part of the original controversy. If your air flow is restricted, you cannot pull as much air per unit time, which directly correlates to power loss in a stoichiometric engine.

 

[Thecardoc]

One thing for sure there are as many different ways to create driver command inputs to the PCM (powertrain control module) as "you engineers" (VBG) can think of, and as I a tech I have to understand and react to every one of them.

Most of the cars in the last ten years use a fairly linear TPS sensor. While inside the PCM it may well work in steps to a degree, as a tech with a scan tool I see both throttle position sensor voltage as well as throttle opening percentages. Most of the cars learn a closed throttle position on key on, and unless there is an issue that causes a TPS voltage that is lower than the original closed throttle, the closed throttle position will always report a 0-1%. (99.9 percent of the time) The one thing the the PCM does with the TPS more than anything else is respond to sudden movement. That's why a poor connection, or a sensor that is wearing out and having voltage drops/spikes drives the system crazy. The biggest thing about OBDII is that it isn't a diagnostic system for technicians, it's an emissions program. The vehicle by design is supposed to adjust itself to get the lowest emissions possible, and then test to make sure it's working. To achieve the lowest possible emissions, it needs to keep air fuel ratios in tight check. (Long term trim, short term trim, and rear trim) and then continue the adjustments to reduce fuel consumption and increase fuel mileage as much as possible, VCT (variable cam timing), EST (electronic spark timing).

Now spark timing maps, they are all over the place as far as what input has more authority over another between engine platforms. The thing is, no-one else even considered "fly by wire", or computer controlled throttle actuators. Between the computer having complete direct throttle control, MAF (mass air flow) sensor inputs, RPM, CTS (coolant temperature) , VVCT (variable cam timing), KS, (knock sensor), it only goes to figure that the PCM will learn, and then continue to re-learn as the filter gets dirtier. What makes this neat is you could have positive feedback for any actual change in the required throttle position for any type of a cruise demand as you played around with different air filter restrictions.

So can you guess where this is going?

It is not at all beyond comprehension that the PCM's programming could be written to alert a technician about whether an air filter actually required replacement or not. Everything is already there, it's just a matter of writing the code.

 

[JWaterstreet]

Excellent thread. Here's my low-tech anecdote that supports the conclusion reached by the group. I've a Toyota Landcruiser that I use to haul the family to our summer house on a lake over 200 miles away. It's stored over the winter. I always fill up at the same two gas stations at either end. I'm particular with logging MPG, which has averaged 13.4 MPG over a couple of years. Before putting it away two winters ago, I changed oil & all filters (including air) Last summer, brought it out, and it seemed a bit sluggish on it's first trip of the summer. Checked MPG; showed 13.6 on the way down, 13.5 on the way back. Same thing on the next trip. Finally got around to sniffing out the source of the sluggish operation. Opened up air filter box, and found it virtually plugged with a huge rodent nest. From the density of the nest, I struggled to understand how the engine produced as much power as it did. However, I rechecked my fuel log, and could not discern any noticeable change in fuel economy, with an almost totally plugged filter. Luckily, the mice did not chew through the filter, so nothing nasty made it's way into the engine.

Jeff

 

[dicer]

With Carburetor yes. With EFI don't think so.
At first I thought TPS reports throttle position, but it is still the O2 sensor that is the final judge and along with the MAF sensor. The HP will suffer. This situation is just like driving your EFI car to the top of a mountain say 12,000 foot altitude. Its going to adjust for the lack of air.

 

[Thecardoc]

Hi Dicer.

When we first started dealing with feedback carburetor systems and the early fuel injection systems we were taught that the O2 sensor makes the computer change the air fuel ratio. The people that told us that is how it works either didn't really understand the system themselves, or else lied. VBG..

The computer uses either a MAF system or a MAP system (speed/density) to calculate a base fuel pulse width. Then it looks to the O2 sensor (Air/Fuel) to see if it is reporting rich or lean. The computer then adds or subtracts fuel from is base calculation (Short term fuel trim) and watches for the O2 sensor (Air/Fuel) to react. This process continues until there is sufficient time for a long term trim number to be learned, where it causes the PCM to make the fuel correction right as it makes it's initial base calculation. Watching a scan tool like the Ford IDS really puts it into perspective how fast short term trim moves, and then you see the individual O2 sensors react to the short term changes.

I won't get into rear fuel trim right now, it really gets complicated, let alone alcohol compensation (E85 systems). Just remember the O2 sensor does not make the computer change the pulse width. The computer changes the pulse width and watches for the O2 sensor (or Air/Fuel) to react.

 

[JSteve2]

Thecardoc,
In the controls universe, the O2 sensor is just operating as a "feedback" controller rather than as a "feedforward" controller. You can't have perfect control with feedback because you must have a disturbance before you can adjust. However, even though it works as you describe, that would be considered as the O2 sensor controlling it in controls lingo. Therefore, I think that the people who told you how it works understood it, but just didn't want to give an entire controls lecture.

 

[SonyAD]

I think a partially clogged air filter will certainly effect maximum torque through the rev range on any engine, including modern, ECU managed engines.

The rest of the time the ECU will probably just open the throttle plate more or not need to do anything on a turbodiesel. I don't see the O2 sensor innate feedback loop delay as a significant cause of mpg drop with a clogged air filter. Certainly not in high driving, for example.

The engine still has to work against the same pressure gradient (sump versus cyl. above piston crown and intake manifold). Only it's due more to a restriction in the air filter than before and less in the throttle plate.

 

[patprimmer]

Sony

I think wealready said that.

Can't you sleep.

Regards

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